Network Powered (+/−190 Vdc or +/−130 Vdc) equipment applications are subject to various restrictions on the twisted pair cables (22AWG/24AWG or similar) typically used in Telecom Applications. The input source is also restricted to voltage, current and power levels (i.e. +/−190V+/−3%, 250-265 mA and 100 VA) thus impeding the total available loop length for higher powered applications.
In particular, the current limitation represents some unique issues with respect to charging input bulk capacitance and controlling fault conditions which could collapse the input source. Most input inrush solutions are based on low voltage or more common −48V Telecom inputs. Typical controllers or discrete solutions limit the peak current on a single startup attempt and are not restricted to the amount of current provided by the input source. In the case for Network Powered applications, a large bulk capacitance cannot be charged with a constant current source method as this would incur excessive losses in the input power switch and undesirable time constraints. By pulse charging in stages, at just under the peak current limitation, the input source is not affected. Also, for fault conditions, the power switch will be destroyed if it is stressed outside of its Safe Operating Area (SOA). Typically hiccup current limiting has to be adopted here to provide a single solution for both operating conditions. The step-charging method, by design, also provides this feature.
Existing solutions may depend on fuses (fault conditions or ptc/thermistors (charging) to manage current drawn from the input source. Only under true abnormal conditions should the fuse be relied upon to safely limit the undesirable effects of a fault. Over temperature, passive devices such as thermistors (NTC), used for inrush current control, are unreliable and their trip thresholds vary over temperature. Since a SEM must operate in adverse working environments and is subject to radical change in internal temperatures, these devices are not applicable for consistent performance. Many PTC, aka resettable fuse, devices are also not rated for the high voltages associated (>200 Vdc) with the input section of the SEM.
For inrush control, common −48V ICs are available but are not rated for this voltage level and do not provide the pulse charging feature during startup nor do they provide the programmable hiccup timer exactly as desired. Also, 12V and 48V/60V sources are typically high current power supplies and able to provide peak startup/inrush currents in/around 3 A or more in typical 100 W applications. For the latter, the highest charge to be applied to an input bulk capacitance is 75V which is ⅕ the requirement here. For Network powered applications, the available charge currents are limited (250 mA) for charging large bulk capacitances to higher voltages extending to 400V. In order to not current limit the source, charge the capacitance in a reasonable amount of time, and not exceed the Safe Operating Area of the pass-FET, a step-charging method with peak fault-current monitoring is needed.
Therefore, it would be desirable to have an inrush power control scheme that take into account both these critical resources of surge current and peak demand.